The paper selection tab allows you to change the paper in use on the printer. Most of the information on this screen does not change the report layout and therefore does not require a regeneration of the report. Even so, changing the paper or orientation may make the report unusable without regeneration or modification in the designer (e.g. the report lines may go past the end of the page).
The one item that does change the layout significantly is the number of columns. Without a regeneration, changing the number of columns would typically mean the report lines are truncated and unreadable.
The paper size should be the same as that found in your printer. The page orientation will depend on your needs and can be portrait or landscape.
Choose the unit of measure with which you are most comfortable and familiar. The report layout does not change, just the numbers shown to you on this screen will change. This can be millimeters or inches.
The margins specify the amount of white space you want around the edge of the page, and so dictates the amount of room for the report. Increasing the left margin will move the report to the right of the paper, and so can be used to centre the report in the middle of the page.
The number of columns can be 1, 2, 3 or 4. This is most useful where the amount of room on the page far exceeds the width of the line of information you wish to print. The type of report will influence the choice here.